Alkali metal doped cobalt and manga-
nese fluoride paramagnetic materials
for wave energy amplification



United States Patent .ALKALI METAL DOPED COBALT AND MANGA- This invention relates to single crystal materials capable of amplifying wave energy by stimulated emission of radiation, and more particularly to crystal materials of the type described for amplifying wave energy in the microwave region.

Although not limited thereto, the present invention is particularly concerned with masers which, generally speaking, may be defined as devices for amplifying or generating microwave energy by utilizing molecules in the excited state of a microwave transition. Interaction between these excited molecules and a microwave field produces additional radiation and hence amplification by stimulated emission. The operation of masers is dependent upon the fact that in paramagnetic materials the electrons surrounding the nucleus of an atom may be in different energy states. These energy states may be thought of as arising from the interaction of the electron spins within internal or external fields. We may, therefore, refer to them as electron spin states. The energies of the electron spin state-s may be varied by an external magnetic field. Thus, the energy difference between two given electron spin states is determined by the magnitude of the external magentic field.

If a paramagnetic material having an excess electron spin population in a higher energy state is placed in a resonant cavity, and if electromagnetic energy of appropriate frequency is fed into the cavity, the electron spins in the higher energy state will revert to a lower energy state, thereby releasing energy which amplifies the signal fed into the cavity. Thus, the two energy states of electron spins in the paramagnetic material may be identified as E and E respectively. Part. of the electronspins in the paramagnetic material will be in the lower energy state E while the remainder will be at the higher energy state E The paramagnetic material is in thermal equilibrium, corresponding to a normal or relaxed condition, when there is an excess electron spin population in the lower energy state E Electron spins in the energy states E and E can interact with a microwave radiation field of appropriate frequency and either absorb energy from the radiation field while advancing to a state of greater energy or, under the influence of the radiation field, can give up some of their energy and drop to a state of lower energy. The amount of energy thus transferred (i.e., (E E is related to the frequency of the radiation field by the following equation:

E E =hV where h is Plancks constant and V is the frequency.

If an electron spin in the lower energy state E is placed in a microwave field of appropriate frequency, it will absorb energy and advance to the upper energy state E An electron spin in the upper energy state E on the other hand, can give up energy to the microwave field and drop to the lower energy state E The probability for either transition is the same. Therefore, whether a system of many electrons exhibits a net absorption or emission of energy depends upon whether more electrons are in the lower or upper energy state. All such systems when allowed to come to thermal equilibrium (i.e., are relaxed) have more electron spins in the lower energy state E and, hence, are absorptive.

In order for the paramagnetic material to be emissive and release energy, there must be an excess of electron spins in the upper energy state E Such an excess population in the upper state may be produced in various ways well known in the art; however, the present invention is not concerned with apparatus for achieving this function. Rather, the invention is concerned with the paramagnetic materials which are used in masers and the like and which contain elements having electron spins capable of existing at different energy levels.

Specifically, the present invention resides in the discovery that single crystals formed from a host material comprising a transition or rare earth element halide doped with an element selected from Group I or Group II of the periodic table makes an excellent paramagnetic material for use in maser and the like applications. In providing any material suitable for maser applications, it is considered desirable to have one with a broad absorption band and a narrow emission band. That is, the band of frequencies at which the material will absorb energy to raise the electron spins from state E to E mentioned above, should be as wide as possible. On the other hand, the band of frequencies at which electrons release energy and drop from state E to state E should be as narrow as possible to minimize the amount of power required. It has been found, in general, that the materials of the invention are particularly suitable in this regard in that they can be made to have broad absorption and narrow emission bands and can be made to amplify wave energies of different frequencies.

The transition or rare earth element halides from which the single crystals may be formed include, without limitation, the following:

Chromium Cobalt Iron Manganese Nickel Titanium Vanadium Any rare earth halide other than the dopant halide. In the manufacture of the single crystals, any one of the foregoing host materials is doped with an element, preferably a metal selected from Group I or Group H of the periodic table, the element being added to the host material in the manufacture thereof preferably as a halide. Such elements include:

Hydrogen Sodium Lithium Potassium Copper Rubidium Cesium Silver Gold Beryllium Magnesium Calcium Strontium Barium Zinc In manufacturing the crystals, the host material selected from the foregoing transition or rare earth element halides and about 0.005 to 5 percent by weight of the dopant element, or combination of dopant elements, preferably but not exclusively in the form of a halide, are mixed together and heated in a crucible or mold of graphite or other refractory material in an evacuated chamber to the melting temperature of the mixture, the mold being withdrawn from the heating chamber gradually, usually over a period of days, whereby a single crystal will grow from a seed at one end of the mold progressing toward the other end as the mold is withdrawn from the heating chamber. Alternatively, instead of withdrawing the mold containing the mixture from the heating chamber, the crystal may be grown by withdrawing it gradually from a molten pool, starting from a seed, in accordance with well known crystalagrowing practices. In either case, however, the crystal, whether it be in the mold or otherwise, must be withdrawn from the heating zone at a certain critical rate.

If an attempt were made to grow the crystals by simply mixing the host material selected from the aforesaid transition or rare earth element halides with a dopant halide and heating the mixture, the resulting product would be very impure, because oxides formed during the heating process would contaminate the crystals. Accordingly, a leaching agent, preferably of the type shown and described in my copending application Serial No. 189,302 filed April 23, 1962, now Patent No. 3,203,899, is mixed with the host material and the dopant element halide and the mixture of the three constituents placed in the aforesaid refractory crucible. The leaching agent must be such that it will readily combine with excess water and other impurities and boil off or volatilize in the evacuated chamber at a temperature lower than the melting point of the host material. At the same time, the leaching agent must be such that it will not combine with the dopant element addition as otherwise the amount of dopant 1n the final crystal cannot be accurately controlled. Preferably, a leaching agent is select-ed which satisfies the above requirements and which boils off or volatilizes beneath the melting point of the host material, which melting point is usually above 1000 C.

It has been found that in order to grow crystals of this type which are suitable for maser and the like applications, the mold containing the crystal, or the crystal itself, must be withdrawn from the heating zone at a rate of 3 to 6 inches per day, .and preferably 4 inches per day, the resulting crystal comprising the final product without any further annealing or other treatment.

As a specific example, a crystal of cobalt fluoride containing potassium may 'be produced by mixing 132 grams of cobalt fluoride with 2 grams of a leaching agent and 6 grams of potassium fluoride and placed in a cylindrical graphite mold or crucible having a diameter in the range of about 7 to inch. Thereafter, the mixture is heated to 1240 C. in a vacuum whereby the leaching agent, combined with excess water and oxides, boils off at about 200 C. to 500 C. during the heat-up period. After the mixture is melted within the crucible, it is withdrawn from the heating zone at a rate of 4 inches per day, the resulting crystal being an excellent paramagnetic material for use in master applications.

As another example, grams of manganese fluoride may be thoroughly mixed with 2 grams of a leaching agent and 2.5 grams of lithium fluoride and placed in an elongated mold or crucible of the type described above. The mixture is thereafter heated to a temperature of 1000 C. to 1050 C. in a vacoum, and the mold withdrawn from the heating zone at a rate of 4 inches per day to form a single pure crystal having excellent paramagnetic characteristics in that it is capable of amplifying micro Wave energy by stimulated emission of radiation.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in composition and method steps may be made to suit requirements Without departing from the spirit and scope of the invent-ion.

I claim as my invention:

1. A material in single crystal form capable of amplifying wave energy by stimulated emission of radiation consisting essentially of cob-alt fluoride doped with about 0.005 to 5 percent by weight of potassium.

2. A material in single crystal form capable of amplifying wave energy by stimulated emission of radiation consisting essentially of manganese fluoride doped with about 0.005 to 5 percent by weight of lithium.

References Cited by the Examiner UNITED STATES PATENTS 2,945,744 7/1960 Knox 252-62.5 X 3,009,774 11/1961 Linz 2388 OTHER REFERENCES Damon: Maser etc, Aviation Week, Aug. 19, 1957 (pp. 76, 77, 81, 82, 87, and 89).

Mellor: A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans, Green & Co., New York, vol. V of 1924, page 638; vol. 12 of 1932, pages 82-83; and vol. 14 of 1932, pages 32-35.

TOBIAS E. LEVOW, Primary Examiner.

MAURICE A. B'RINDISI, Examiner.

S. R. BRESCH, R. D. EDMONDS,

Assistant Examiners. 

1. A MATERIAL IN SINGLE CRYSTAL FORM CAPABLE OF AMPLIFYING WAVE ENERBY BY STIMULTATED EMISSION OF RADIATION CONSISTING ESSENTIALLY OF COBALT FLUORIDE DOPED WITH ABOUT 0.005 TO 5 PERCENT BY WEIGHT OF POTASSIUM.
 2. A MATERIAL IN SINGLE CRYSTAL FORM CAPABLE OF AMPLIFYING WAVE ENERGY BY STIMULATED EMISSION OF RADIATION CONSISTING ESSENTIALLY OF MANGANESE FLUORIDE DOPED WITH ABOUT 0.005 TO 5 PERCENT BY WEIGHT OF LITHIUM. 